The lighting theory and structure of light-emitting diode (LED) is different from that of conventional lighting source. LED has advantages as a low power loss, a long life-time, no need for warm-up time, and a short responsive time. Moreover, it is small, shockproof, suitable for mass production, and highly compatible with applications demanding a tiny or array-type element, so LEDs are widely adopted in the market. For example, LEDs can be used in optical display apparatus, laser diodes, traffic lights, data storage devices, communication devices, illumination devices, medical devices, and so on.
Nevertheless, because the refractive index of an LED is greater than that of air, most of the emitted light thereof is reflected back inside from the interface between semiconductor layer and air. The efficiency of output of power (actual output of power/nominal power of the light source) of different materials is within a range of 2.21% to 4.18% without any additional processing.
Recently, a roughening process on the surface of the semiconductor layer or the substrate of an LED has been adopted, so the probability of the light output of the light which would be scattered because of the total reflection in conventional structure is increased. For other LEDs, the original substrate for epitaxial-growth is removed and the semiconductor layer is bonded with another substrate having a large size so that the substrate having a reflective layer at the bottom increases the probability of the light output by providing another chance for the light to be reflected.